RULES in binary

[Lennart Kolmodin]

Yes, this has been on my todo for a long time :)
Essentially all inlinings/rules in binary should be gone through and
confirmed whether they're still needed.
I had a look now to get some insight.

Since a few versions GHC warns in this way when something might not go the
way it was intended, a great way to learn more about how inlining and rules
work and to avoid surprises.
All these warnings are proof of my poor understanding when I implemented
it. Naturally it should all be fixed.

Here's how I reasoned when implementing it;
In Data.Binary.Get we have functions we always want to inline, even if GHC
doesn't think it's a good idea. Therefore there are both INLINE pragmas as
well as RULES to achieve this. GHC now warns that the function might get
inlined before the rule triggers, which is ok since they do the same thing.
We should probably re-evaluate whether always inlining still is a good
idea. If it is, we can keep the RULES to inline, and change the INLINE to
NOINLINE and let the RULES do their job.

In Data.Binary.Internal.Get we attempt a trick where applicative code can
become more efficient. It tries to rewrite the components of an expression
"f <*> g <*> h" into something that does f, g and h with a single bounds
check (the check for "do we have enough input bytes to continue?").
This trick relies so much on that the user's code has been inlined properly
that it probably very rarely fires in a real application. It does wonders
in the unrealistic micro benchmark, though :)
Probably those rules can be removed without any real code suffering. I'd
like to add some more real world benchmarks, and finally test with the
changes proposed above.

Lennart


2015-07-24 13:44 GMT+02:00 Simon Peyton Jones <simonpj at microsoft.com>:

>  Lennart
>
> In the binary library I’m seeing lots of these warnings:
>
> libraries/binary/src/Data/Binary/Get.hs:420:1: warning:
>
>     Rule "getWord16le/readN" may never fire
>
>       because ‘getWord16le’ might inline first
>
>     Probable fix: add an INLINE[n] or NOINLINE[n] pragma on this function
>
>
>
> libraries/binary/src/Data/Binary/Builder/Base.hs:510:1: warning:
>
>     Rule "flush/flush" may never fire
>
>       because ‘flush’ might inline first
>
>     Probable fix: add an INLINE[n] or NOINLINE[n] pragma on this function
>
> The warnings look right to me: currently everything is very fragile and
> may not work as you intend.
>
> You may want to look into this?
>
> Simon
>
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